1. Field of the Invention
The present invention relates to a data relay unit and a multiplex communication system and, more particularly to a method for controlling relay of a data frame when a failure occurs in the multiplex communication system.
2. Related Art
In recent years, sophistication of communication networks advances as computer technology advances. In the field of motor vehicles, the volume of information exchanged among electronic control units (ECUs) which control respective electrical components installed on a vehicle increases rapidly. Therefore a multiplex communication system is employed for communication among the ECUs in the vehicle so that the number of wire harnesses required for the communication is reduced.
In the multiplex communication system, ECUs are connected as nodes to a common multiplex communication line and communicate each other by exchanging data frames through the common communication line. In the case of the vehicle, all ECUs are appropriately divided into a plurality of groups according to the required communication speed. Thus a plurality of networks corresponding to the respective groups are formed for efficient data communication among the ECUs. Communication between two ECUs belonging to different networks is performed via a data relay unit.
In the data relay unit, terminals to which the respective communication lines are connected and internal circuits which receive data frames via the respective terminals are all concentrated at one place. In recent years, package density is generally increased as the need for miniaturization of devices is increased. Therefore the terminals and inner circuits of the data relay unit are also arranged so as to be close to each other. Accordingly, there is a possibility that the communication lines of the different networks are shorted due to foreign matter or water which enters the system. When such short occurs, a data frame which has been relayed to the destination network by the data relay unit would appear on the communication line of the sender network again. In this case, the data frame is relayed by the data relay unit again, and this is repeated endlessly. As a result, communication load is increased rapidly.
EP0886404A2 (JP-A-H11-8653) discloses a system which includes a plurality of data relay units and communication lines. The communication lines connect the data relay units so that a mesh network is formed. In this system, each node sets time to live (TTL) in a data frame when it sends the data frame to the destination node. If the data frame cannot be sent to the destination node due to a failure of one of the communication lines, a new route is established. However, the data frame would be relayed repeatedly until the new route is established. Accordingly, this system discards the data frame when the TTL of the data frame reaches, so that the data frame is prevented from being relayed repeatedly for a considerable time.
If this technique is employed to solve the above problem in the present multiplex communication system, each node is required to include a section for setting the TTL of a data frame. Further this technique which is devised for the mesh network is not cost-effective in this case, because the mesh network is not formed in the multiplex communication system.